1. Field of the Invention
The present invention relates to a motor driving system for a brushless motor, in particular to an improvement of the motor driving system for a brushless motor suitable for a spindle motor of a video tape recorder (VTR), a disc memory device, and for a spindle motor for driving a polygon mirror.
2. Description of the Related Art
FIG. 5 shows a sectional side view of a brushless motor for driving directly a capstan shaft of a VTR.
FIG. 6 shows a plan view of a stator of the brushless motor shown in FIG. 5.
A rotor 30 is fixed to a shaft 40, a capstan shaft, which is held to rotate by a bearing 100 and a spindle 90. The rotor 30 is provided with a drive magnet 50, facing to a stator 20. The drive magnet 50 has eight magnetic poles magnetized in a form of sine-wave. Around the drive magnet 50, an FG (frequency generator) magnet 60 having 360 magnetic poles is provided.
The stator 20 is comprised of a stator substrate 70 and six stator coils 80 of coreless assemblies. The stator substrate 70 is made of a soft magnetic steel sheet having an insulating layer and an electronic circuit thereon. The electronic circuit is formed by etching a copper layer on the insulating layer. The six stator coils 80 of coreless assemblies are disposed at 60 degrees apart around the center of the shaft 40 respectively, and fixed to the stator substrate 70. The stator coils 80 face the drive magnet 50. Respective two of the stator coils 80, opposed each other across the spindle 90, are wired in a series connection, and compose three phases of stator coils of a star connection as a whole. Two Hall elements HGU and HGV are disposed at a center of stator coils 80 which are separated at 120 degrees apart from each other as shown in FIG. 6. The Hall elements HGU, HGV output Hall signals U, V of Hall voltages respectively, in response to a magnetic flux of the drive magnet 50 as it rotates.
A magnetic sensor of magnetoresistance (MR) element (not shown) is placed on the stator 20, and face an outer edge of the FG (frequency generator) magnet 60 across a gap of 0.1 mm. The FG magnet 60 is provided on the rotor 30. The magnetic sensor outputs an FG signal of 360 Hz per revolution. The FG signals are used as speed control signals of a brushless motor, and supplied to a drive circuit contained in an integlated circuit (IC) 140, for supplying a drive current to a stator coil 80.
The Hall signals U and V are added together and then reversed the phase thereof to form a compound signal W.
A waveform of the Hall signal U (V) is depended on a waveform of a magnetization of the drive magnet 50. When the drive magnet 50 is magnetized in a sine wave, the wave form of the Hall signal U (V) becomes a sine wave. The Hall signal U, V and the compound signal W form three phases of positional signals, and differ 120 degrees in electric phase angle each other. These three phases of positional signals are supplied to the drive circuit. The drive current, subjected to the FG signals, is switched by the three phases of signals U, V and W. The drive circuit supplies the switched drive current to the respective stator coils 80. A revolving magnetic field is produced by the switched drive current fed to the stator coils 80, and rotates the rotor 30.
The compound signal W, provided by adding the Hall signals U, V, and then by reversing the phase of the added signal, is required to have a same waveform of the Hall signal U (V), and to have a small phase error referred to respective phases of the Hall signals U and V. When the Hall signals U, V of sine wave differs in the phase of 120 degrees, the compound signal W can be made to a sine wave and to have the phase of 120 degrees from those of the Hall signals U, V respectively as shown in FIG. 1.
In general, amplitudes of the Hall signals U, V differ each other, as a Hall element has a sensitivity variation. Due to the amplitude difference, the phase of the compound signal W is shifted in relation to the Hall signals U and V as shown in FIG. 2. This phase shift of the compound signal W causes a timing shift of switching of the drive current supplied to the stator coil 80, and an irregularity of rotation increases. When the Hall signals U and V are made to a trapezoidal wave to generate a large torque from the brushless motor, a waveform of the compound signal W becomes a triangular shape. This triangular shape of the compound signal W can be corrected into a corrected compound signal W' of trapezoidal wave by a nonlinear circuit. But, when there is a difference between the amplitudes of the Hall signals U and V, the waveform of the corrected compound signal W' becomes different from those of the Hall signals U, V, and causes a phase shift as shown in FIG. 3. For this reason, the larger the difference between the amplitudes of the Hall signals U and V becomes, the larger the irregularity of rotation becomes as shown in FIG. 4.